This invention relates to a polarized beam coupler for coupling two light beams having polarization planes that are perpendicular to each other, and more particularly to the polarized beam coupler suitable for producing an output beam to be monitored.
In the past, dual light sources have been used in an attempt to construct a highly reliable optical fiber communications system. In such system the two light sources are constructed such that the two emergent light beams may be coupled to an optical transmission line. At an initial stage of operation of the system, only one of the light sources is used. If the initial stage light source should fail, the other light source is used to prevent a system shut down which would otherwise take place. A polarized beam coupler is used in a highly reliable system of this type to couple the emergent light beams from two light sources to a common optical transmission line. In such systems it is necessary to split a part of the emergent light beam to monitor the same in order to supervise, for example, age deterioration of the light source. Accordingly, a polarized beam splitter is demanded which produces an emergent light beam which can be well monitored.
FIG. 1 is a schematic illustration showing the general construction of a conventional polarized beam coupler. Referring to FIG. 1, the polarized beam coupler includes a polarized beam coupling cube 11 having a polarized beam coupling film 11a formed from a dielectric multi-layer film or the like. By using a cube of such construction, two incident light beams having polarization planes that are perpendicular to each other can be projected on the same optical axis. In the device of FIG. 1, a P wave from a first input port has a polarization plane which is an oscillation plane of an electric field vector and which is parallel to the plane of incidence of the polarized beam coupling film 11a so that the P wave passes through the polarization beam coupling film 11a. On the other hand, an S wave from a second input port has a polarization plane which is perpendicular to the plane of incidence of the polarized beam coupling film 11a so that the S wave is reflected by the polarized beam coupling film 11a, and thus the transmitted light and the reflected light emerge on the same optical axis.
The polarized beam coupler further includes a beam splitting cube 12 for extracting a monitor beam from one of the beams emerging from the polarized beam coupling cube 11 on the same optical axis. The beam splitting cube 12 normally includes a beam splitting film 12a formed from alternating layers of metal and SiO.sub.2 in order to suppress the polarization plane dependency. The light beams which are incident upon the beam splitting cube 12 and which pass through the beam splitting film 12a are introduced into an optical transmission line from an output port for a signal beam. However, the portion of the light beams incident upon the beam splitting cube 12 and which are reflected by the beam splitting film 12a are introduced into a suitable photo-detector from a monitor beam output port so that the same may be used for monitoring the output beam level, feedback control and so forth.
When the beam splitting film 12a includes a metal film, the absorption loss at the beam splitting film 12a may be so great that, when a monitor beam of a predetermined level is extracted, the level of the signal beam coupled to an optical transmission line is diminished. In consideration of this problem, it may seem recommendable to employ, for such beam splitting film 12a, a dielectric multi-layer film composed of alternating layers of Al.sub.2 O.sub.3 and TiO.sub.2. In such case, however, distinct from the case wherein the beam splitting film includes a metal film, the polarization dependency of the transmitting power of the beam splitting film 12a is extremely high at an angle of 45 degrees, an angle which is otherwise advantageous as an angle of incidence to the beam splitting film 12a in constructing an optical device as shown in FIG. 2. Accordingly, where a dielectric multi-layer film is employed as the beam splitting film, a construction as shown in FIG. 3 has been adopted.
To reduce the polarization dependency of the transmitting power of the beam splitting film, the conventional device of FIG. 3 employs a beam splitting prism 13 wherein the angle of incidence to the beam splitting film 13a may be as small as 10 degrees or so. Thus, a monitor beam reflected by and extracted from the beam splitting film 13a is further reflected by a total reflection face 13b of the beam splitting prism 13 in order to direct the monitor beam to the outside of the prism. The reason why the total reflection face 13b is used to reflect the monitor beam in this manner is that the monitor beam is introduced into the beam splitting film 13a at an angle which is nearly a right angle and if it is not reflected away by the total reflection face 13b, then the beam reflected by the beam splitting film 13a will be intercepted by the polarized beam coupling cube 11a so that it cannot be monitored by a photo-director. Accordingly, a conventional device having such a construction as described above has problems in that the structure of the beam splitting prism is complicated and cumbersome adjustment of the optical axis is required.
As described hereinabove, where a beam splitting film is formed from a dielectric multi-layer film, it is commonly used at an angular disposition where it does not have a polarization dependency. In a conventional device of such construction, the amount of light in a monitor beam extracted from an incident light beam from a first input port and the amount of the light in a monitor beam extracted from an incident light beam from a second input port present a ratio of about 1:1, and it is difficult to change the ratio to an arbitrary value.